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Leo Rising

Leo, the lion, is springing into the evening sky. It’s in full view in the east by about 8 p.m., and its brightest stars form a pattern that really does resemble a crouching lion.

Leo’s brightest star is Regulus. It shines white, with perhaps a slight hint of blue.

All stars produce all wavelengths of visible light, from red to yellow to blue. But based on its surface temperature, each star produces more energy at some wavelengths than at others. Regulus, for example, is hotter than the Sun, so it peaks at the blue end of the spectrum. In fact, Regulus is so hot that it puts out quite a bit of light in the ultraviolet, which is invisible to the human eye.

The Sun is thousands of degrees cooler than Regulus, so it peaks at yellow wavelengths, which are in the middle of the spectrum of visible light. That’s why the Sun is sometimes described as a yellow star.

But a star’s color becomes obvious only from a great distance. At close range, the colors all blend together, generally making the star look white — just as the Sun does in our sky. If we could see the Sun from a few light-years away, though, it would show a definite yellowish tint.

Once a star gets really far away, though, so that it becomes only a tiny pinpoint in the sky, it looks white. That’s not because of a change in the star itself, but because it’s not bright enough to trigger the color receptors in our eyes — in effect, bleaching away the star’s true character.

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